Hydrogen energy is considered as a highly efficient and clean energy source that has received worldwide attention and become one of the important development directions of new energy sources. The best route to obtain hydrogen is first to generate electricity using low-cost renewable energies, followed by producing hydrogen by proton exchange membrane water electrolysis (PEMWE). The most efficient device that utilizes hydrogen is a fuel cell, especially a proton exchange membrane fuel cell (PEMFC), and one of the key materials for the PEMWE and PEMFC is the electrocatalyst. A proton exchange membrane (PEM) provides an acidic electrolyte environment for PEMWE- and PEMFC-related electrode reaction processes including hydrogen and oxygen electrode reactions, and both theoretical decomposition voltage and cell voltage are 1.23 V. Under such conditions, in addition to meeting the requirements of high conductivity and catalytic activity, the electrocatalyst must have high corrosion resistance and have good electrochemical stability within the potential range of electrocatalysis reactions. Up to now, the electrocatalysts meeting the above requirements are mainly platinum group noble metals and their alloys and oxides. Based on this existing technical situation, worldwide material scientists and chemists are facing demanding challenges of preparing highly efficient, stable, low-cost electrocatalyst with a simple and easy method.
Currently, a mainstream in the water electrolysis field is PEMWE technology. Such technology has resulted in an increase in efficiency from 70% for traditional electrolysis to 85% or more, greatly reducing the energy consumption. At present, a main limitation to the industrial application of PEMWE technology is the high cost of materials. The proton exchange membrane now can be produced in a mass batch, and thus the expense is expected to be reduced to an acceptable level. Accordingly, a key to cost reduction is focused on the choice and preparation process of the electrocatalyst for hydrogen and oxygen evolution. The hydrogen-evolution catalyst for PEMWE mainly belongs to the Pt group, and the cathode is treated with a noble metal oxide (such as RuO2, IrO2, etc.) to avoid Pt poisoning and inactivation caused by an underpotential deposition. Among these, RuxTi1-xO2 shows excellent resistance to chloride and iron ions. The best known oxygen-evolution catalyst for PEMWE is IrO2. Marshall et al., (Marshall A, BØrresen B, Hagen G, et al. Electrochimica Acta, 2006, 51(15): 3161-3167) applied IrO2 as the oxygen evolution catalyst for PEMWE. In their work, it was reported that, under an electrolysis temperature of 90° C. and working current density of 1 A/cm2, its cell voltage is 1.61V, indicating an outstanding oxygen-evolution electrocatalyst. However, the expense of using pure IrO2 is too high and there is room for further electro-catalytic performance improvement.
The oxygen reduction electrocatalyst for PEMFC is the most essential factor concerning cell performance and cost, which needs about 0.4 V overpotential in cathodic polarization, and uses a large amount of noble metal catalyst resulting in a low noble metal utilization rate. Pt and its alloy supported on carbon is the only electrocatalyst showing some activity for oxygen reduction at low temperature and in acidic electrolyte. Pt/C produced by E-Tek Inc. of USA has been used by a number of institutes and companies all over the world, due to its excellent electrocatalytic performance for cathode oxygen reduction. However, many studies have shown that, after long-term operation, some phenomena related to a weak interaction between Pt and C occur, such as transfer, aggregation and corrosion of Pt. Besides this, the corrosion of the carbon support was also found. To solve these problems, Halalay and Merzougui et al. used titanium nitride, titanium carbide or titanium dioxide to support platinum and its alloys to prepare a catalyst in US20060246344 A1 and WO2006119407, respectively. However, this method brought a problem of poor catalyst conductivity, and therefore, they added carbon powder or titanium nitride and titanium carbide with good conductivity into the catalyst during the preparation of a membrane electrode to enhance the conductivity in the above two patents. Results show that the conductivity of the support is poor and the nanoscale titanium nitride and titanium carbide powder has significantly reduced conductivity and anti-oxidation properties, which has a negative impact on the long-term stability of the catalyst. In addition, inclusion of conductive powder that has no catalytic activity in the membrane electrode would inevitably reduce the effective catalytic active surface.
Because a noble metal of the platinum group has to be used in the catalyst design, as little as possible should be used while still ensuring catalytic activity. Other concerns such as conductivity, high specific surface area, stability and ease of preparation of a nano-powder should also be considered. The preparation method has a significant influence on the catalytic properties, amount of supported noble metal and noble metal efficiency of the catalyst. At present, the preparation methods of nano-powder catalysts mainly include the physical vapor deposition method, chemical vapor deposition method, homogeneous precipitation method, direct precipitation method, sol-gel method, organic complex precursor method, hydro-thermal synthesis method, micro-emulsion method, solid phase method, coating method, radiation chemical synthesis method, electrochemical precipitation method, Adams melt casting method, impregnation-liquid phase reduction method, vapor reduction method, high temperature alloying method, microwave thermal synthesis method and metal-organic compound thermal decomposition method, and so on. These methods have been widely used to give various kinds of nano-powder catalysts. However, although great development has been achieved, it is hard to satisfy the comprehensive properties for nano-powder catalyst by current preparation methods.
Therefore, the urgent problems in this field include: using non-noble metals to form multiple composite oxides with different electrochemical properties, so as to reduce the amount of the noble metal; developing a new catalyst or catalyst support with excellent conductivity, corrosion-resistance and anti-oxidation properties to replace conventional carbon materials; and developing a new method for preparing a nano-powder catalyst at low cost, under the premises of using a low amount of noble metal and providing excellent electrocatalytic properties and long term stability.